The present invention generally relates to variable-frequency electrical power systems and, more specifically, to a method and apparatus for automatically transferring electrical power between an auxiliary power unit (APU) generator and a main engine generator in such variable-frequency systems.
An APU is a gas turbine engine which can be used to provide electrical power and air conditioning on board an aircraft, as well as for starting a main engine. When the aircraft is being serviced on the ground, an electrical bus in the aircraft electrical system may be connected to a ground-support power supply to provide necessary electrical power to aircraft components. The ground support power supply may be a portable ground cart or a fixed facility power supply. When the aircraft prepares to leave the ground support facility, the source of electrical power may be transferred from the ground-support power supply to an electrical generator powered by the APU in the aircraft. In some applications, ground support power may not be used, and the APU electrical generator may be the sole source of electrical power.
Subsequently, when the aircraft prepares for departure, APU power may be used to start the aircraft main engines. Once the main engines are operating, the source of electrical power may be transferred from the functioning APU electrical generator (i.e., the load-dropping generator) to a main engine electrical generator (i.e., the load-receiving generator). After landing, electrical power may be transferred in the reverse sequence, from the main engine electrical generator to the APU electrical generator, and then from APU electrical generator to the ground support power supply.
As can be appreciated by one skilled in the relevant art, a “no-break” (i.e., uninterrupted) power transfer (NBPT) is preferred when there are power-sensitive components, such as volatile computer memory, which would be affected by a temporary loss of electrical power if the functioning electrical power source is disconnected before a replacement electrical power source is connected. Accordingly, when a “break power transfer” is performed instead of a no-break power transfer, data entered into volatile computer memory, for example, would need to be re-entered. A break power transfer is an acceptable operating sequence, but is less desirable than a no-break power transfer.
In most aircraft which use AC power systems instead of DC systems, the rotational speed of the main engine electrical generator is controlled such that the output power has an essentially constant frequency. Controlling the rotational speed of the main engine electrical generator may be accomplished, for example, by utilizing a mechanical speed-control device between the engine gearbox and the generator. The APU is normally controlled as a fixed-speed engine and therefore the APU electrical generator frequency is also essentially constant. When executing a no-break power transfer, the frequency and other parameters of the load-receiving and load-dropping generators must be closely matched in order to prevent damage to the generators or to the drive train. With constant frequency electrical systems, this operation is relatively straightforward. Prior to the power transfer, the difference in frequency between the APU electrical generator and the main engine electrical generator will be relatively small, since both are controlled to be constant-frequency generators. Additionally, a “target frequency” signal, essentially equivalent to the frequency of the main engine electrical generator, may be sent to an APU electronic control unit which can adjust fuel flow to the APU until the target frequency is realized by the APU electrical generator.
In the present state of the art, some aircraft electrical systems operate with variable-frequency electrical power and control of the rotational speed and output frequency of the main generator may be provided over a wide range of frequencies and speeds. Accordingly, these newer systems may employ a variable-speed APU also having a variable, but more limited, operating frequency range. With the new variable frequency system, designing the APU to transfer a full electrical load at any matching frequency within the entire ground operating frequency range of the main engine electrical generator would significantly penalize the size, weight, and cost of the APU. That is, the APU would have to be larger, heavier, and costlier to be able to accept full load at minimum speed. The APU would also require additional structure and control methods to minimize speed transients and meet containment safety requirements in order to be able to transfer (drop) full load at maximum speed. An APU designed to accommodate the wide frequency range of the main engine generator, and to allow for the worst possible combinations of air pressure (ambient altitude), load, and temperature conditions would be a larger and heavier unit.
U.S. Pat. No. 5,729,059, issued to Kilroy et al., discloses the digital transmission of electrical parameters to minimize wiring between electrical components of a no-break power transfer system. However, in the system taught by Kilroy et al. '059, the APU is capable of power transfers, a typical feature in fixed frequency electrical systems. Accordingly, the system taught by Kilroy et al. '059 does not address variable frequency electrical systems in which a substantial frequency mismatch between load-transferring generators may be present. In the present invention, communication between the aircraft electrical power system and the auxiliary power unit system is implemented such that no-break power transfers may be rendered practical in applications where some of the load-transferring generators operate over a broad frequency range.
As can be seen, there is a need to maintain the ability to execute no-break-power-transfers in variable-frequency electrical systems during most of the normally encountered operating conditions without adding significant size, weight, and cost to the APU.